Make sure managesieve_default is not a dir (#6773)

plugins/managesieve/config.inc.php.dist

@@ -41,7 +41,7 @@ $config['managesieve_usetls'] = false;
 // Note: These can be also specified as an array of options indexed by hostname
 $config['managesieve_conn_options'] = null;
 
-// default contents of filters script (eg. default spam filter)
+// A file with default script content (eg. spam filter)
 $config['managesieve_default'] = '/etc/dovecot/sieve/global';
 
 // The name of the script which will be used when there's no user script

plugins/managesieve/lib/Roundcube/rcube_sieve_engine.php

@@ -258,7 +258,7 @@ class rcube_sieve_engine
                     $script_name = 'roundcube';
                 }
 
-                if ($script_file && is_readable($script_file)) {
+                if ($script_file && is_readable($script_file) && !is_dir($script_file)) {
                     $content = file_get_contents($script_file);
                 }